fix(stdio): release ReadBuffer backing after final byte

[piyushbag]

Problem

ReadBuffer.readMessage() leaves an empty Buffer subarray after consuming a complete line. The subarray still references the accumulated backing allocation, so idle stdio transports retain memory longer than necessary and the next append always copies via Buffer.concat() even when no bytes remain buffered.

Approach

When the remainder after a newline is empty, set _buffer to undefined instead of an empty subarray. Non-empty remainders are unchanged.

Test plan

• Added regression test asserting Buffer.concat is not called on the next append after a fully consumed message
• pnpm --filter @modelcontextprotocol/core-internal test (1294 passed)

Fixes #2536

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commit: 8a36bde